Various vehicles are equipped with smart control systems. Such smart control system performs locking and unlocking of a vehicle door through a verification of an identification (ID) of an electronic key (i.e., portable identification device). That is, a request signal is transmitted from a transmitter provided at a vehicle to the electronic key, and, in response to the request signal, a return signal is transmitted from the electronic key to the vehicle. The smart control system is convenient because the vehicle door is automatically unlocked when a user carrying the electronic key approaches the vehicle door.
Besides the smart control system, various systems such as a tire pressure monitoring system are mounted to a vehicle so as to enhance convenience and safety of a passenger of the vehicle. The tire pressure monitoring system includes air pressure sensors provided at respective tires and each including transmitting and receiving function. Then, on the basis of measurement information (i.e., detection result of the air pressure sensor) sent by each of the air pressure sensors and received by a receiver provided at the vehicle, the tire pressure monitoring system detects an abnormality of any tires and indicates a warning at an indicator.
In cases where various systems are mounted to the vehicle, some system components are desirably shared between the systems so that the number of parts as a whole vehicle is reduced, which leads to low cost systems. JP3636184B discloses a system in which transmitters are shared between the smart control system and the tire pressure monitoring system.
In order to monitor the tire pressure, it is generally required to identify from which air pressure sensor, specifically, tire, of the vehicle the measurement information is sent. Such identification of the measurement information is achieved by preliminary correlating positions of the tires in the vehicle to identifications (IDs) assigned to the air pressure sensors provided at the respective tires. At this time, transmitters for sending ID transmission request signals to the respective air pressure sensors are required so that the ID is transmitted from each of the air pressure sensors. According to the system disclosed in JP3636184B, transmitters designated to the smart control system are also used for sending the ID transmission request signals in the tire pressure monitoring system. Each of the transmitters for the smart control system has a transmission area that includes only one tire provided in the vicinity of each door of the vehicle. For example, the ID transmitted from the transmitter having the transmission area including the right front tire can relate to the right front tire.
Upon consideration of sharing system components between the systems, on the contrary to the case described in JP3636184B, transmitters designated to the tire pressure monitoring system to transmit the ID transmission request signals to the respective air pressure sensors may be used for the smart control system. However, the transmission areas of the transmitters, each specified to include only one tire to identify the positions of the tires in the vehicle, are not appropriate and not sufficient for locking and unlocking the vehicle doors with the smart control system.
Accordingly, it may be considered to expand the transmission area as large as possible while including only one tire. However, an unlocking allowable area by the electronic key is also enlarged, which is undesirable in view of safety concerns.
Such problem is not limited to a case where the transmitters for the tire pressure monitoring system are used for the smart control system and may occur when a mounted position of each of the transmitters for the smart control system has restrictions.
A need thus exists for a communication system for a vehicle which is not susceptible to the drawback mentioned above.